This invention relates to a transmitter in a fiber optic system. The transmitter utilizes a transmission line that is configured to achieve optimized impedance matching without use of an impedance matching network.
Fiber optic systems generally have three main components, a transmitter, a transmission medium, and a receiver. Fiber optic systems use light pulses to transmit information down fiber lines, which are then received and generally translated to electrical signals. Optical receivers generally receive and convert a modulated light signal coming from the optical fiber back into a replica of the original signal, which was applied to the transmitter.
A transmitter generally includes driver circuit and an optical emitter that are electrically coupled. The optical emitter can be a laser or LED. The driver circuit receives a modulated electrical signal that contains information that is to be transmitted over the optical fiber in the form of a modulated optical signal. The driver circuit is coupled to the laser or LED and is configured to cause the light-emitting device to generate a modulated optical signal based upon the modulated electrical signal.
Modern day fiber optic systems are required to be operated at increasingly high frequency rates. The frequency of the electrical signal sent from the driver circuit to the light emitter is often so high that the signal acts like a wave. Accordingly, one important consideration for driver circuits in driving light emitters in the transmitters of the fiber optic system is impedance matching of the elements. If the output of the driver circuit has different impedance than does the light emitter, signal reflections will occur. Signal reflections disturb the standing-wave oscillation and cause intersymbol interference in the light emitter that can cause significant intolerable error in the fiber optic transmission system.
In order to compensate for mismatched impedance, most transmitters also include an impedance matching network that can interface the output of the driver circuit with the light emitter such that the impedance will appear matched from both the light emitter and from the driver circuit. Typically a light emitter load like a laser will have lower impedance than the output of the driver circuit. Consequently, a typical matching network will include a plurality of resistive elements that will deflect some of the signal from the light emitter so that the impedance matching and there will be no reflections.
Unfortunately, these matching networks cause significant wasted energy in the system and are often difficult to place where required due to geometric restrictions. Because part of the signal goes through these matching networks so that impedance will be well matched, portions of the signal are typically going though resistors in the matching network that are parallel with the load. Some of this diverted current will release energy as heat, which is wasted energy in the system. Energy from the diverted current in the matching network that is not released as heat can instead generate electromagnetic interference, which can cause additional problems for other parts of the system.